The North American Testis Workshop has been held for nearly twenty years. This Workshop has, until this year, been funded by contracts from NIH. Each of the Workshops has focused on various facets of testicular function. These Workshops have served as the focus for information exchange on testicular function for two decades and have been perceived by most of the participants as the major meeting in their field. Recent rapid and dramatic developments in our understanding of the underlying genomic expression and control that takes place during spermatogenesis and sperm maturation and fertilization have led the Program Committee to focus the 1991 Workshop on selective aspects of the male germ cell as it progresses form being a spermatogonium through the complex phases of spermatogenesis and sperm maturation to fertilization. The recent breakthroughs in our understanding of how steroid hormones act via their receptors and the fascinating similarity amongst steroid hormone receptors will first be presented. This will be followed by the genetic control of spermatogenesis and the genomic determination of what makes a male a male. The complex fine tuning of the hormonal milieu that allows spermatogenesis to take place will then be presented. Exciting developments have occurred in our understanding of the structure and function of organelles specific to spermatozoa as well as specific gene expression during spermatogenesis. How germ cells relate to their environment during spermatogenesis (germ-Sertoli cell interactions) and during epididymal transit will then be discussed. Finally, the application of the technical breakthroughs in our understanding of sperm motility and of the underlying processes of sperm-egg interaction will be presented. The format of all presentations will be a 25 minute talk followed by a thorough 20 minute discussion of each paper. The long coffee and lunch breaks will help promote further communication amongst the participants. Two poster sessions that will be given in conjunction with wine and cheese sessions will allow all participants to present voluntary contributions of their most recent research findings.